This invention relates to a novel epoxy resin composition to be employed as a structural substrate of printed circuit boards. Due to that the trend of fact the electronic devices is characterized by one toward small size, high density and high reliability in recent years, the requirements of printed circuit board substrate thus tend to be of high thermal resistance and electrical insulation. The substrate material for printed circuit boards now in use, however, known as FR-4, with a glass transition temperature (Tg) of about 110.degree. C. can not meet the above requirements. Though the printed circuit board based on bis-maleimide, the so called addition type polyimide, has very high thermal resistance, it has not yet replaced FR-4 PCB consumer products because of its high cost. Therefore, epoxy resin with high Tg provides a potential solution to the problems because of its low price and excellent electrical properties.
In prior literature and patents, there are many reports concerning the improvement of electrical processing, thermal and mechanical properties on epoxy based printed circuit boards. However, the improvement was achieved at the expense of some other properties. The main prior art patents include U.S. Pat. Nos. 4,559,395, 4,645,803, 4,550,051, 4,529,790, 4,594,291, 2,947,726, 2,971,942, and 2,809,942. According to U.S. Pat. No. 4,559,395, an effective amount of amine having an aryl ring in structure, and 2.5-10 wt. % of dicyandiamide, is added to a mixture of two epoxy resins, hence a cured product with Tg at about 150.degree. C. is obtained. In U.S. Pat. No. 4,594,291, the bisphenol-A epoxy resin is modified by a chain extender with two active hydrogens. Then an acidic or amine type curing agent or curing promoter is added to form a thermal resistant (i.e. high glass transition temperature) resin composition. In those reports, the epoxy resins adopted for printed circuit boards are all multifunctional epoxy resins, such as novolac expoxy resin, tetrakis (hydroxy phenyl) ethane, or tetraglycidyl ether, to increase the degree of cross linking. However, the prior arts reported in these literature have the following defects:
1. With the degree of cross linking increased, the hardness will also be raised. Therefore, the life of the drill heads will be decreased so that operating cost will be increased.
2. As the degree of cross linking increases, the stiffness or toughness of cured products and the adhesion to copper clad will decrease.
3. Though the stiffness or toughness of cured products could be enhanced by the addition of chain-extended bisphenol-A epoxy resin with higher molecular weight, the employment of such epoxy resins would also have serious negative effects on glass transition temperature (Tg) and result in the formation of smears during the drilling process.